In radiography, and particularly in medical radiography, light-sensitive elements having silver halide emulsion layers coated on both faces of a transparent support (called double-side coated silver halide elements) are used. Said double coated silver halide elements are generally used in combination with fluorescent phosphor screens in order to reduce the X radiation exposure necessary to obtain the required image. Generally, one fluorescent phosphor screen is used in association with each silver halide emulsion layer of the double coated element. The silver halide emulsions used in the double coated element are sensitized to a region of the electromagnetic spectrum corresponding to the wave length of the light emitted by the phosphor materials used in the fluorescent phosphor screens, thus obtaining a significant amplification factor.
It is known that in radiography the silver halide radiographic elements are exposed in radiographic cassettes consisting substantially of a base element, generally permeable to X-rays, and an upper element, generally permeable to X-rays, which are joined together by a vertical perimetral element so as to form a container having an essentially flat parallelepiped shape. Said flat horizontal upper and base elements and said vertical element have the minimum dimensions necessary for containing a radiographic film of a certain format disposed between intensifying screens fixed to said upper and base elements. Said screens are uniformly pressed against the film so as to ensure good uniform contact with the film, as known in the art. Said base element is generally constructed in the form of a lid hinged along the rear edge of the cassette. Radiographic cassettes are described, for example, in U.S. Pat. Nos. 3,323,676, 3,504,180, 4,264,821, 4,352,198, 4,630,297, and the European patent applications Nos. 18,564 and 26,358.
The quality of the image obtained upon X radiation exposure of said screen pair and double coated silver halide element combination and development of said double coated silver halide element is negatively affected by crossover exposure. Crossover exposure, which causes a reduction in image sharpness, occurs in double coated silver halide elements when light emitted by one fluorescent phosphor screen passes through the adjacent silver halide emulsion layer and, the light having been spread by the support, imagewise exposes the silver halide emulsion layer on the opposite face of the support.
The crossover exposure causes poor definition even if light-sensitive elements are used which employ reduced silver halide coverages to lower the costs or increase the processing speed of the element. In fact, the decrease of the emulsion turbidity increases the amount of light available for crossover and therefore worsens the image.
To reduce the crossover exposure, dyes or pigments can be used within the radiographic element. The absorption of said dyes or pigments is in a region of the electromagnetic spectrum corresponding to the wavelength of the light emitted by the fluorescent phosphor screens. The dyes or pigments absorb some of the light emitted by the fluorescent phosphor screen so that imaging of a silver halide emulsion layer by the opposite screen is reduced by absorbance of the light from the opposite screen by the anticrossover dyes or pigments. These dyes or pigments are eliminated during the photographic developing, fixing and washing process of the exposed material; they can be for instance washed away or, more preferably, bleached while processing the radiographic element.
The dyes can be incorporated in any layer of the light-sensitive element: in the emulsion layer, in an intermediate layer between the emulsion and the base, or in the subbing layer of the support base. It is preferred to incorporate the dyes in a layer different from that containing the emulsion to avoid possible desensitization phenomena. Since 1978, Minnesota Mining and Manufacturing Company has sold a radiographic element under the name of 3M Trimax.TM. Type XUD X-Ray Film to be used in combination with 3M Trimax.TM. Intensifying Screens. Such radiographic element comprises a transparent polyester base, each surface of which has a silver halide emulsion layer sensitized to the light emitted by the screens. Between the emulsion and the base is a gelatin layer containing water-soluble acid dyes, which dyes can be decolorized during processing and have an absorption in a region of the electromagnetic spectrum corresponding to the wavelength of the light emitted by the screens and of the spectral sensitivity of the emulsion. The dyes are anchored in the layer by means of a basic mordant consisting of polyvinylpyridine.
In the practical solution of reducing the crossover exposure by using a mordanted dye layer (as described for instance in the European Patent Application 101,295), some problems are created which up to now have not yet been solved properly. In fact, the improvement of image definition involves not only a natural decrease of the light-sensitive element sensitivity caused by the absorption of the transmitted light which otherwise would take part in the formation of a part of the image, but also the possibility of desensitization phenomena due to the migration of dye not firmly mordanted in the silver halide emulsion layer. There is also a problem with residual stain even after processing, the retention of significant quantities of thiosulfate from the fixing bath which causes image Yellowing upon long-time storage on shelf, and lengthening of the drying times after processing because of element thickening.
Other approaches have been suggested to reduce crossover, as reported hereinafter.
U.S. Pat. No. 3,923,515 discloses a relatively lower speed silver halide emulsion between the support and a higher speed silver halide emulsion layer to reduce crossover.
U.S. Pat. No. 4,639,411 discloses a photographic element, to be used with blue emitting intensifying screens, having reduced crossover, said element comprising coated on both sides or a transparent support a blue sensitive silver halide emulsion layer and, interposed between the support and the emulsion layer, a blue absorbing layer comprising bright yellow silver iodide grains of a specific crystal structure.
Japanese Patent Application 62-52546 discloses a radiographic element of improved image quality comprising coated on both sides of a transparent support a light sensitive silver halide emulsion layer and, interposed between the support and the emulsion layer, a layer containing water insoluble metal salt particles having adsorbed on their surface a dye. Said dye has a maximum absorption within the range of .+-.20 nm of the maximum absorption of said silver halide and corresponds to the light emitted by intensifying screens. Silver halides are disclosed as preferred metal salt particles.
Japanese Patent Application 62-99748 discloses a radiographic element of improved image quality comprising coated on both sides of a transparent support a light-sensitive silver halide emulsion layer and, interposed between the support and the emulsion layer, a silver halide emulsion layer having substantially no light-sensitivity.
The approaches of using light-insensitive silver halide layers as anticrossover layers interposed between the support and the light-sensitive silver halide emulsion layers, although preferred to using dyes or pigments, encounter some problems such as the increase of silver coverage and bad bleaching characteristics in photographic processing (residual stain).
The following are additional documents illustrating the state of the art.
BE 757,815 discloses a combination of a silver halide element and an intensifying screen comprising a fluorescent compound emitting light of wavelength less than 410 nm.
U.S. Pat. No. 4,130,428 discloses a combination of two fluorescent screens and a double coated silver halide element wherein the maximum emission of the screens is in the wavelength range of 450-570 nm and silver halide layers are sensitive to light in the same wavelength range.
U.S. Pat. No. 3,795,814, 4,070,583 and GB 2,119,396 disclose rare earth oxyhalide phosphors activated with terbium and/or thulium employed in fluorescent screens having UV emission.
FR 2,264,306 discloses a combination of a silver halide element and fluorescent screen comprising a rare earth activated rare earth oxyhalide phosphor having it maximum emission in the wavelength range of 400-500 nm.
EP 88,820 discloses a radiographic fluorescent screen comprising a first blue emitting phosphor layer and a second green emitting phosphor layer to be combined with a silver halide element having spectral sensitivity in the blue-green region ("ortho-type" elements).
JP 60175-000 discloses a combination of a double coated silver halide element and a screen pair wherein the fluorescent layers of the two screens have different wavelength region emissions and each screen comprises an organic dye to absorb the light emitted by the opposite screen.
EP 232,888 discloses a combination of a double coated silver halide element and a pair of front and back intensifying screens wherein said front and back screens, emitting light in the same wavelength region, have different modulation transfer factors to be used in low energy radiography.
U.S. Pat. No. 4,480,024 discloses the combination of a silver halide photothermographic element and a rare-earth intensifying screen which are uniquely adapted to one another for the purpose of industrial radiographic imaging. The photothermographic element is dye-sensitized to the spectral emission of the screen and the combination of screen and element has an amplification factor greater or equal to at least 50. According to this invention preferably a single screen is used in combination with a single-side coated photothermographic element , or double screens with either single-side or a double-side coated photothermographic elements, the latter without any significant benefit and at increased cost of film.